1. Field of the Invention
The present invention relates to a laser printer. More particularly, the present invention relates to a halftone processing method for improving print quality by correcting the variation in light intensity on a photoconductive drum, and an apparatus using the same.
2. Description of the Related Art
Printers are currently the most popular device for producing the results from a personal computer (PC) into hard copies for verifying, maintaining and transmitting, and are a fundamental component of multi-function printers, facsimiles, electronic cash registers, and automatic teller machines (ATMs).
Modern ink jet printers and laser printers have replaced earlier daisy wheel printers, dot matrix printers and line printers to become two of the most popular types of printers.
A laser printer uses physical elements such as toner, a light beam (as provided by a laser or LED array), electrostatic attraction, heat, and pressure to produce desired results. The core technique of the laser printing method is electrostatic attraction.
FIG. 1 is a schematic structural diagram of a conventional laser printer. Referring to FIG. 1, the laser printer includes a printing unit 100, which prints an image onto a paper S, an output path 240, a reversal path 250, a paper feed cassette 200, a pickup roller 201, and a feed roller 210.
The printing unit 100 includes a charger 120, a laser scanning unit (LSU) 130, a photoconductive drum 110, developers 140 filled with developing agents, a transfer belt 150, a transfer roller 160, and a fixing unit 170. The printing unit 100 prints an image onto a paper S using electrophotography. The printing unit 100 can print a color image and thus, includes four developers 140 that are filled with developing agents for black (K), cyan (C), magenta (M), and yellow (Y) respectively.
A procedure of forming an image using the printing unit 100 will now be briefly described. First, the charger 120 supplies an electric charge to the photoconductive drum 110 to thereby charge the photoconductive drum 110. Next, an exposure is performed to form an electrostatic latent image on the photoconductive drum 110. If the LSU 130 scans light which corresponds to information regarding a yellow color for example, onto the photoconductive drum 110, a yellow electrostatic latent image is formed due to the differences in electric potential between portions where light was scanned and other portions. Next, the developer 140 supplies developing agent to the electrostatic latent image to develop the electrostatic latent image and form a yellow toner image. The toner image is then transferred to the transfer belt 150.
After the transfer of the yellow toner image to the transfer belt 150, magenta, cyan, and black toner images are sequentially transferred to the transfer belt 150 using the same method as described above, superimposing these toner images with the yellow toner image. As a result, a color toner image is formed on the transfer belt 150. The color toner image is then transferred onto paper S passing between the transfer belt 150 and the transfer roller 160, and heat and pressure are applied to the paper to fix the color toner image onto the paper. Accordingly, a color image is obtained.
FIG. 2 is a detailed structural diagram of the LSU 130 of FIG. 1. The LSU 130 forms an electrostatic latent image by scanning an optical signal, such as a laser beam, over a photoconductive medium, such as a photoconductive drum 310.
Referring to FIG. 2, the LSU 130 includes a light source 307, a rotating polygon mirror 309 driven by a motor (not shown) for reflecting a laser beam emitted from the light source 307, an f-θ lens 315 for focusing the laser beam reflected by the rotary polygon mirror 309 onto the surface of the photoconductive drum 310 to form a spot with an appropriate diameter along a scanning line 318, and a reflecting mirror 320 located along an optical path between the f-θ lens 315 and the photoconductive drum 310 to reflect an incident beam so that the laser beam passing through the f-θ lens 315 is directed toward the photoconductive drum 310. An electrostatic latent image is formed on the photoconductive drum 310 by switching the light source 307 on and off.
A collimating lens 322 is provided for converting an incident beam into a parallel beam, and a cylindrical lens 335 is provided for converging the parallel beam onto a reflective surface of the rotary polygon mirror 309, and are both located along the optical path between the light source 307 and the rotary polygon mirror 309. A laser beam detector 325 is further provided and equipped to detect the position where the scanning line 318 starts.
Here, the laser beam emitted from the light source 307 is converted into the parallel beam by the collimating lens 322, and the parallel beam passes through the cylindrical lens 335 and is reflected by the rotary polygon mirror 309. The beam reflected by the rotary polygon mirror 309 passes through the f-θ lens 315, and the reflecting mirror 320 changes the optical path of the beam so that the beam is focused on the photoconductive drum 310 to form the spot at a point along the scanning line 318 of the photoconductive drum 310.
In a laser printer, a phenomenon can occur wherein the resolution is inferior at the edges of the paper than in the center, since the intensity of the laser beam is lower at the edges of the photoconductive drum 310 than in the center.
Referring to FIG. 3, the incident angle of a laser beam being incident to the rotary polygon mirror 309 varies according to the rotation of the rotary polygon mirror 309. In general, the reflection rate of the rotary polygon mirror 309 is best when the incident angle is 45° and gradually decreases as the incident angle is deviated from the angle of 45°. Accordingly, the intensity of the laser beam is the strongest at the center of the photoconductive drum 310 and weakens towards the edges.
The reflection ratio of the f-θ lens 315 also affects the variation of the light intensity on the photoconductive drum 310, since the reflection rate of the edges of the f-θ lens 315 is inferior to that at the center.
FIG. 3 is a graph obtained by measuring the intensity of receiving beams on the photoconductive drum 310. In FIG. 3, the horizontal axis indicates the distance from the center of the photoconductive drum 310, and the vertical axis indicates the intensity of beams received at the corresponding positions. Individual plots of the graph indicate the intensity of beams measured with respect to different LSUs #2, #3, #4 and #5, respectively. As shown in FIG. 3, the intensity of the beams is strongest at the center of the photoconductive drum 310 and weaker towards the edges. Specifically, the intensity of the beams is detected at a value of 0.3 mW at the center of the photoconductive drum 310, and at a value of 0.255 mW to 0.272 mW at the edges. The plots of the graph shown in FIG. 3 are obtained by measuring the intensity of a receiving beam on the photoconductive drum 310 using an optical power meter.
The variation in the intensity of the beams on the photoconductive drum 310 causes uneven printing quality or resolution deterioration at the edges of the paper.
Accordingly, a need exists for a system and method for correcting the variation in the intensity of a beam generated on a photoconductive drum.